The prior art for some time has recognized the need for manhole extender means to raise the level of the cast iron manhole cover to a new pavement level when streets are resurfaced. Commonly, a metal extender ring of the necessary thickness is welded to the manhole top ring to raise the level of the manhole cover, as required. Other prior art proposals include adjustable manhole extenders such as screw-threaded extender rings. All of these proposals will accomplish the desired result, but generally speaking they are costly to manufacture or require a good deal of labor for installation, particularly in the case of welded rings. The disadvantages of these prior art extender rings is that welded joints break under heavy traffic and mechanical joints loosen under vibration. Some examples of the patented prior art are shown in the following U.S. Pat. Nos. 1,639,495, 3,408,778, 1,908,909, 3,490,177, 2,903,875, 3,533,199, 3,385,011, 3,629,981, 3,968,600.
Even with the improvements of the above discussed patents, the common way today to raise the level of a manhole cover is through use of shims beneath the ground ring. This, of course, requires that the dirt around the ring be excavated to permit installation of the shims and return of the dirt after such installation.
The objective of this invention is, therefore, to provide a better and more economical means for extending manholes in order to elevate the manhole cover to a new level when street paving takes place. The product of the invention is constructed of inexpensive material and lends itself to more economical manufacturing processes. It also provides a better seat for the manhole cover and is comparatively silent when vehicles pass over the manhole cover. Another advantage of the invention is that the plastic extender ring can be trimmed or tailored in the field to provide a better fit on the manhole base ring and with the cover. The extender ring can be cast as a unitary annular ring or extruded and then cut to size or can be molded in several segments which are joined or seated during usage by caulking or the like. The product is extremely tough and wear-resistant.
Other features and advantages of the invention will become apparent during the course of the following description.